Lithographic mask, lithographic apparatus and method

ABSTRACT

A lithographic mask has a substrate substantially transmissive for radiation of a certain wavelength, the substrate having a radiation absorbing material in an arrangement, the arrangement configured to apply a pattern to a cross-section of a radiation beam of the certain wavelength, wherein the absorbing material has a thickness which is substantially equal to the certain wavelength divided by a refractive index of the absorbing material.

This application is a continuation application of U.S. patentapplication Ser. No. 13/743,231, filed on Jan. 16, 2013, now allowed,which claims priority and benefit under 35 U.S.C. §119(e) to U.S.Provisional Patent Application No. 61/587,587, filed on Jan. 17, 2012,U.S. Provisional Patent Application No. 61/587,941, filed on Jan. 18,2012 and U.S. Provisional Patent Application No. 61/589,027, filed onJan. 20, 2012. The entire content of each of the foregoing applicationsis incorporated herein by reference.

FIELD

The present invention relates to a lithographic mask, to a lithographicapparatus and to a device manufacturing method.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthat circumstance, a mask, which is alternatively referred to as a maskor a reticle, may be used to generate a circuit pattern corresponding toan individual layer of the IC, and this pattern can be imaged onto atarget portion (e.g. comprising part of, one or several dies) on asubstrate (e.g. a silicon wafer) that has a layer of radiation-sensitivematerial (resist). In general, a single substrate will contain a networkof adjacent target portions that are successively exposed. Knownlithographic apparatus include so-called steppers, in which each targetportion is irradiated by exposing an entire pattern onto the targetportion in one go, and so-called scanners, in which each target portionis irradiated by scanning the pattern through the beam in a givendirection (the “scanning”-direction) while synchronously scanning thesubstrate parallel or anti parallel to this direction.

Progressive improvements of a lithographic apparatus have been madewhich have allowed patterns with smaller resolutions to be projectedonto substrates. One such improvement involves providing a liquidbetween a projection system of a lithographic apparatus and a substrate.This provides the projection system with a numerical aperture (NA)greater than 1.0 (e.g. 1.35NA).

SUMMARY

When a high numerical aperture is used (such as when using immersionlithography), the mask used to pattern radiation may give rise to anunwanted phase effect. Specifically, the topography of the mask (i.e.unevenness across the surface of the mask) may introduce an unwantedphase offset into the patterned radiation. Such a phase offset mayreduce the accuracy with which a pattern is projected onto a substrate.

It is desirable to provide, for example, a mask which obviates ormitigates one or more problems of the art, whether defined herein orelsewhere.

According to an aspect of the invention, there is provided alithographic mask comprising a substrate which is substantiallytransmissive for radiation of a certain (e.g., predetermined)wavelength, the substrate having a radiation absorbing material in anarrangement, the arrangement configured to apply a pattern to across-section of a radiation beam of the certain wavelength, wherein theabsorbing material has a thickness which is substantially equal to thecertain wavelength divided by a refractive index of the absorbingmaterial.

According to a further aspect of the invention, there is provided alithographic mask comprising a substrate which is substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the absorbing material has a thicknessequal to or within a first range of 10% of the radiation wavelengthdivided by a refractive index of the absorbing material.

According to a further aspect of the invention, there is provided alithographic mask comprising a substrate which is substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the absorbing material has a thicknessequal to or within a second range of 20% of the radiation wavelengthdivided by a refractive index of the absorbing material.

The thickness of the absorbing material within the first range or thesecond range may be further chosen depending on a phase shiftcharacteristic and resulting image contrast provided by the absorbingmaterial to radiation of the certain wavelength.

The thickness of the absorbing material may be chosen to optimize thephase shift characteristic and resulting image contrast provided by theabsorbing material to radiation of the certain wavelength.

The absorbing material may comprise a plurality of material layershaving different refractive indices, and the refractive index of theabsorbing material may be taken as being the average of the refractiveindices of the material layers, the average taking into account theproportions of the different materials through which the radiation beampasses.

The lithographic mask may comprise a first arrangement having a firstradiation absorbing material having a first thickness, the firstthickness determined as described above and the lithographic maskfurther comprises a second arrangement having a second radiationabsorbing material having a second thickness, the first thickness andthe second thickness being different.

The first arrangement may be a functional pattern and the secondarrangement may be a measurement pattern.

The first radiation absorbing material may be the same material as thesecond radiation absorbing material.

The certain wavelength may be one of 193 nm, 365 nm, 248 nm, 157 nm or126 nm.

According to a further aspect of the invention, there is provided alithographic mask comprising a reflective substrate having a radiationabsorbing material in an arrangement, the arrangement configured toapply a pattern to a cross-section of a radiation beam of a certainwavelength, wherein the absorbing material has a thickness which issubstantially equal to or a multiple of the certain wavelength dividedby twice a refractive index of the absorbing material, after taking anoffset into account.

According to a further aspect of the invention, there is provided amethod comprising providing a substrate, providing a beam of radiationwith a certain wavelength using an illumination system, using a mask toimpart the radiation beam with a pattern in its cross-section, andprojecting the patterned radiation beam onto a target portion of thesubstrate, wherein the mask comprises a substrate which is transmissiveto the radiation beam and to which a radiation absorbing material isprovided in an arrangement, and wherein the thickness of the absorbingmaterial is substantially equal to the radiation wavelength divided by arefractive index of the absorbing material.

According to a further aspect of the invention, there is provided alithographic mask comprising a substrate which is substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the radiation absorbing materialarrangement comprises a plurality of structures having a first pitch anda plurality of structures having a second pitch, and wherein theabsorbing material has a thickness which is such that the structureshaving the first pitch and the structures having the second pitch willhave substantially equal best focus planes when projected using aprojection system of a lithographic apparatus.

According to a further aspect of the invention, there is provided alithographic mask comprising a substrate which is substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the radiation absorbing materialarrangement comprises a plurality of structures having a first pitch anda plurality of structures having a second pitch, and wherein theabsorbing material has a thickness which is such that the structureshaving the first pitch and the structures having the second pitch willexperience substantially equal image shifts when projected using aprojection system of a lithographic apparatus. The image shifts aredisplacements of the images of the structures after imaging in adirection substantially perpendicular to the optical axis of the imagingsystem.

According to a further aspect of the invention, there is provided alithographic mask comprising a reflective substrate having a radiationabsorbing material in an arrangement, the arrangement configured toapply a pattern to a cross-section of a radiation beam having a certainwavelength, wherein the radiation absorbing material arrangementcomprises a plurality of structures having a first pitch and a pluralityof structures having a second pitch different compared to the firstpitch, and wherein the absorbing material has a thickness which is suchthat a focus difference between the best focus plane for the structureshaving the first pitch and a best focus plane for the structures havingthe second pitch substantially corresponds to a minimum of a variationof the focus difference versus absorbing material thickness whenprojected using a projection system of a lithographic apparatus.

According to a further aspect of the invention, there is provided alithographic mask comprising a reflective substrate having a radiationabsorbing material in an arrangement, the arrangement configured toapply a pattern to a cross-section of a radiation beam of the certainwavelength, wherein the radiation absorbing material arrangementcomprises a plurality of structures having a first pitch and a pluralityof structures having a second pitch different compared to the firstpitch, and wherein the absorbing material has a thickness which is suchthat an image shift difference between the image shift experienced bythe structures having the first pitch and the image shift experienced bythe structures having the second pitch substantially corresponds to aminimum of a variation of the image shift difference versus absorbingmaterial thickness.

The first pitch, as measured after projection by the projection systemof the lithographic apparatus, may be substantially half of thewavelength of the certain wavelength.

The first pitch, as measured after projection by the projection systemof the lithographic apparatus, may be smaller than the certainwavelength and the second pitch, as measured after projection by theprojection system of the lithographic apparatus, may be larger than thecertain wavelength.

According to a further aspect of the invention, there is provided ablank mask comprising a substrate which substantially transmissive forradiation of a certain wavelength, the substrate having radiationabsorbing material, the radiation absorbing material being etchable togenerate a pattern in the radiation absorbing material, wherein athickness of the radiation absorbing material is such that followingetching the radiation absorbing material has a thickness which issubstantially equal to the certain wavelength divided by a refractiveindex of the radiation absorbing material.

According to a further aspect of the invention, there is provided ablank mask comprising a reflective substrate provided with a radiationabsorbing material, the radiation absorbing material being etchable togenerate a pattern in the radiation absorbing material, wherein athickness of the radiation absorbing material is such that followingetching the radiation absorbing material has a thickness whichsubstantially corresponds to a minimum of a variation of the focusdifference versus absorbing material thickness when projected at acertain wavelength using a projection system of a lithographicapparatus.

A layer of a radiation sensitive resist may be provided over theradiation absorbing material.

According to a further aspect of the invention, there is provided amethod of determining a thickness of a radiation absorbing material tobe provided to a lithographic mask, the lithographic mask comprising asubstrate which is substantially transmissive for radiation of a certainwavelength and the substrate comprising the radiation absorbing materialarranged to form a plurality of structures for imaging via a projectionsystem of a lithographic apparatus, the method comprising selecting aplurality of structures having different pitches, determining best focusplanes of the selected structures when imaged via the projection system,the best focus planes being determined for different thicknesses of theradiation absorbing material, and selecting the thickness of theradiation absorbing material for which the plurality of structures havesubstantially equal best focus planes when imaged using the projectionsystem.

According to a further aspect of the invention, there is provided amethod of determining a thickness of a radiation absorbing material tobe provided to a lithographic mask, the lithographic mask comprising asubstrate which is substantially transmissive for radiation of a certainwavelength and the substrate comprising the radiation absorbing materialarranged to form a plurality of structures for imaging via a projectionsystem of a lithographic apparatus, the method comprising selecting aplurality of structures having different pitches, determining imageshifts of the selected structures when imaged via the projection system,the image shifts being determined for different thicknesses of theradiation absorbing material, and selecting the thickness of theradiation absorbing material for which the plurality of structures havesubstantially equal image shifts when imaged using the projectionsystem.

According to a further aspect of the invention there is provided amethod of determining a thickness of a radiation absorbing material tobe provided to a lithographic mask, the lithographic mask comprising areflective substrate and the substrate comprising the radiationabsorbing material arranged to form a plurality of structures forimaging using a projection system of a lithographic apparatus, themethod comprising selecting a plurality of structures having differentpitches, determining best focus planes of the selected structures whenimaged via the projection system using a certain wavelength, the bestfocus planes being determined for different thicknesses of the radiationabsorbing material, and selecting the thickness of the radiationabsorbing material to be a thickness at which a variation, as a functionof absorbing material thickness, of a difference between best focusplanes of the selected structures substantially corresponds to a minimumwhen the structures are imaged using the projection system.

According to a further aspect of the invention there is provided amethod of determining a thickness of a radiation absorbing material tobe provided to a lithographic mask, the lithographic mask comprising areflective substrate and the substrate comprising the radiationabsorbing material arranged to form a plurality of structures forimaging using a projection system of a lithographic apparatus, themethod comprising selecting a plurality of structures having differentpitches, determining image shifts of the selected structures when imagedvia the projection system using a certain wavelength, the image shiftsbeing determined for different thicknesses of the radiation absorbingmaterial, and selecting the thickness of the radiation absorbingmaterial to be a thickness at which a variation, as a function ofabsorbing material thickness, of a difference between image shifts ofthe selected structures substantially corresponds to a minimum when thestructures are imaged using the projection system.

The determining the best focus plane or the determining the image shiftmay be performed by simulating the projection of the structures usingthe projection system.

A first structure in the plurality of structures may comprise a firstpitch, as measured after projection by the projection system, having adimension of substantially half of the wavelength of the certainwavelength.

A first structure in the plurality of structures may comprise a pitchbeing smaller than the certain wavelength, and a second structure in theplurality of structures may comprise a pitch larger than the certainwavelength, the pitches being as measured after projection by theprojection system.

The selecting the thickness may comprise defining a range within whichthe thickness is chosen.

The selecting the thickness may further comprise selecting the thicknessof the absorbing material depending on a phase-shifting characteristicof the absorbing material to the radiation of the certain wavelength.

The thickness of the absorbing material may be further selected tooptimize the phase-shifting characteristic of the absorbing material.

According to a further aspect of the invention, there is providedcomputer program product configured to perform the method according toany of the preceding aspects of the invention.

According to a further aspect of the invention, there is provided alithographic apparatus comprising an illumination system to condition abeam of radiation, a support structure to support a mask, the maskserving to impart the radiation beam with a pattern in itscross-section, a substrate table to hold a substrate, and a projectionsystem to project the patterned radiation beam onto a target portion ofthe substrate, wherein the lithographic apparatus further comprises amask according to any preceding aspect of the invention.

The radiation beam may be a polarized radiation beam.

The polarized radiation beam may have at least two polarization states,an intensity in a first polarization state being different from anintensity in a second polarization state.

According to a further aspect of the invention, there is provided amethod comprising providing a substrate, providing a beam of radiationwith a certain wavelength using an illumination system, using a maskaccording to any preceding aspect of the invention to impart theradiation beam with a pattern in its cross-section, and projecting thepatterned radiation beam onto a target portion of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 schematically depicts a lithographic apparatus according to anembodiment of the invention;

FIG. 2 schematically depicts part of a lithographic mask according to anembodiment of the invention;

FIG. 3 is a graph which shows the results of a simulation which modelsthe best focus of a lithographic apparatus as a function of thethickness of absorbing material on a mask used by the lithographicapparatus;

FIG. 4 is a graph which corresponds with the graph of FIG. 3 but whichsimulates the effect of using a different absorbing material;

FIG. 5 is a graph which corresponds with the graphs of FIG. 3 and FIG. 4but which simulates the effect of using a different absorbing material;

FIG. 6 is a graph which corresponds with the graphs of FIGS. 3-5 butwhich simulates the effect of using two absorbing materials provided inlayers;

FIG. 7 is a graph which shows the results of a simulation which modelsthe best focus of a lithographic apparatus as a function of thethickness of absorbing material of a reflective mask according to anembodiment of the invention;

FIG. 8 is a graph which shows how the range of best focuses varies as afunction of the thickness of the absorbing material; and

FIG. 9 is a graph which corresponds with the graph shown in FIG. 8 butin which the results of the simulation have been divided by thewavelength of the radiation and multiplied by the refractive index ofthe absorbing material.

DETAILED DESCRIPTION

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,liquid-crystal displays (LCDs), thin film magnetic heads, etc. Theskilled artisan will appreciate that, in the context of such alternativeapplications, any use of the terms “wafer” or “die” herein may beconsidered as synonymous with the more general terms “substrate” or“target portion”, respectively. The substrate referred to herein may beprocessed, before or after exposure, in for example a track (a tool thattypically applies a layer of resist to a substrate and develops theexposed resist) or a metrology or inspection tool. Where applicable, thedisclosure herein may be applied to such and other substrate processingtools. Further, the substrate may be processed more than once, forexample in order to create a multi-layer IC, so that the term substrateused herein may also refer to a substrate that already contains multipleprocessed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultra-violet (EUV) radiation (e.g. having a wavelength in the range of5-20 nm, which may for example be 13.5 nm or 6.7 nm).

The support structure holds the mask (which may also be referred to as areticle). It holds the mask in a way depending on the orientation of themask, the design of the lithographic apparatus, and other conditions,such as for example whether or not the mask is held in a vacuumenvironment. The support can use mechanical clamping, vacuum, or otherclamping techniques, for example electrostatic clamping under vacuumconditions. The support structure may be a frame or a table, forexample, which may be fixed or movable as required and which may ensurethat the mask is at a desired position, for example with respect to theprojection system.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “projection lens” herein may beconsidered as synonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents for directing, shaping, or controlling the beam of radiation,and such components may also be referred to below, collectively orsingularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more support structures). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion techniques are well known in the artfor increasing the numerical aperture of projection systems.

FIG. 1 schematically depicts a lithographic apparatus according to aparticular embodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL to condition a beam PB        of radiation (e.g. UV radiation).    -   a support structure (e.g. a support structure) MT to support a        mask MA and connected to a first positioning device PM to        accurately position the mask with respect to item PL;    -   a substrate table (e.g. a wafer table) WT to hold a substrate        (e.g. a resist coated wafer) W and connected to a second        positioning device PW to accurately position the substrate with        respect to item PL; and    -   a projection system (e.g. a refractive projection lens) PL        configured to image a pattern imparted to the radiation beam PB        by mask MA onto a target portion C (e.g. comprising one or more        dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a reflective mask).

The illuminator IL receives a beam of radiation from a radiation sourceSO. The source and the lithographic apparatus may be separate entities,for example when the source is an excimer laser. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation beam is passed from the source SO to the illuminator ILwith the aid of a beam delivery system BD comprising for examplesuitable directing mirrors and/or a beam expander. In other cases thesource may be integral part of the apparatus, for example when thesource is a mercury lamp. The source SO and the illuminator IL, togetherwith the beam delivery system BD if required, may be referred to as aradiation system.

The illuminator IL may comprise adjusting means AM for adjusting theangular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator ILgenerally comprises various other components, such as an integrator INand a condenser CO. The illuminator provides a conditioned beam ofradiation PB, having a desired uniformity and intensity distribution inits cross section.

The radiation beam PB is incident on the mask, which is held on thesupport structure MT. Having traversed the mask MA, the beam PB passesthrough the projection system PL, which focuses the beam onto a targetportion C of the substrate W. With the aid of the second positioningdevice PW and position sensor IF (e.g. an interferometric device), thesubstrate table WT can be moved accurately, e.g. so as to positiondifferent target portions C in the path of the beam PB. Similarly, thefirst positioning device PM and another position sensor (which is notexplicitly depicted in FIG. 1) can be used to accurately position themask MA with respect to the path of the beam PB, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe object tables MT and WT will be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the positioning device PM and PW.However, in the case of a stepper (as opposed to a scanner) the supportstructure MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in the following preferred modes:

1. In step mode, the support structure MT and the substrate table WT arekept essentially stationary, while an entire pattern imparted to thebeam PB is projected onto a target portion C in one go (i.e. a singlestatic exposure). The substrate table WT is then shifted in the X and/orY direction so that a different target portion C can be exposed. In stepmode, the maximum size of the exposure field limits the size of thetarget portion C imaged in a single static exposure.2. In scan mode, the support structure MT and the substrate table WT arescanned synchronously while a pattern imparted to the beam PB isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the supportstructure MT is determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

When imaging a pattern from the mask MA onto the substrate W using alithographic apparatus it may be desirable to position the substratesuch that a focussed image of the mask is formed on the substrate.However, the topography of the surface of the mask may introduce a phasedifference into the radiation when it is patterned by the mask (theheight of the surface of the mask may, for example, vary by around 2-3nanometers). The effect of such a phase difference may be that the bestfocus for an image of a first type of feature (e.g. dense lines) lies ina first plane and the best focus for an image of a second type offeature (e.g. isolated lines) lies in a second plane which is differentfrom the first plane. Thus, it may not be possible to position thesubstrate at a plane in which the best focus is achieved for both thefirst and second types of features. This problem may be referred to asfocus difference. The focus difference may be considered to be theseparation between the first plane and the second plane. Focusdifference may also occur between pattern features having a first pitchand pattern features having a second pitch. Focus difference caused bythe topography of the surface of the mask may be referred to as a 3Dmask effect.

The term “best focus” may be interpreted as meaning the plane in whichthe best contrast of an aerial image is seen. In some instances the bestfocus may be measured by measuring the aerial image directly (e.g. usinga sensor). In some instances the best focus may be measured by imaging apattern onto a substrate a plurality of times, the substrate beingpositioned in a different plane each time the pattern is imaged onto it.A critical dimension of the pattern may be measured for each image ofthe pattern, and this may be plotted as a function of substrate planeposition in order to generate a Bossung curve. A maximum or minimum ofthe Bossung curve may be taken as indicating the best focus of thepattern.

FIG. 2 schematically shows in cross section part of a mask MA accordingto an embodiment of the invention. The mask MA comprises a substrate 1and an absorbing material 2. The substrate 1 may be, for example, formedfrom glass or any other suitable material which is substantiallytransparent to the radiation beam PB of the lithographic apparatus (e.g.DUV radiation). The absorbing material 2 may be, for example, molybdenumsilicide (MoSi) or any other suitable material which absorbs theradiation beam PB of the lithographic apparatus (e.g. DUV radiation) orwhich absorbs part of the radiation beam PB. The MoSi may be providedwith one or more dopants which may modify the refractive index of theMoSi. The absorbing material 2 does not fully cover the substrate 1, butinstead is configured as an arrangement, i.e., pattern. Thus, gaps 3 arepresent between areas of absorbing material 2. Only a small part of themask MA is shown in FIG. 2. In practice the absorbing material 2 andgaps 3 are arranged to form an arrangement which may for example havethousands or millions of features.

The radiation beam PB of the lithographic apparatus (see FIG. 1) isincident upon the mask MA. The radiation beam PB is initially incidentupon the substrate 1 and passes through the substrate 1. The radiationbeam is then incident upon the absorbing material 2 and gaps 3.Radiation which is incident upon the absorbing material 2 passes throughthe absorbing material but is partially absorbed by the absorbingmaterial. Alternatively, the radiation is substantially fully absorbedin the absorbing material 2 and substantially no radiation istransmitted through the absorbing material 2. Radiation which isincident upon the gaps 3 passes through the gaps without beingsignificantly or partially absorbed. The mask MA thus applies a patternto the radiation beam PB (which pattern may be applied to an unpatternedradiation beam PB or applied to a radiation beam PB already having apattern).

The thickness T of the absorbing material 2 is substantially equal tothe wavelength of the radiation beam PB as it travels through theabsorbing material (i.e., taking into account the refractive index ofthe absorbing material). In an embodiment the wavelength of theradiation beam PB may be 193 nanometers. Thus, the thickness of theabsorbing material 2 in nanometers may be 193/n, where n is therefractive index of the absorbing material

193 nanometers is a widely used radiation wavelength in lithographicapparatus. It is conventionally used in immersion lithography apparatus,i.e., apparatus in which a fluid such as water is located between theprojection system PS of the lithographic apparatus and the substrate W.The fluid can provide the projection system with a numerical aperturegreater than 1.0 (e.g. 1.35 NA). When such a high numerical aperture isused, the topography of a mask MA (i.e. unevenness across the surface ofthe mask) may introduce an unwanted phase offset into the patternedradiation, and such a phase offset may cause focus difference. The focusdifference may reduce the accuracy with which a pattern is projectedonto a substrate by the lithographic apparatus. The focus difference isreduced or eliminated when the thickness of the absorbing material 2 isequal to the wavelength of the radiation beam as it travels through theabsorbing material. This may provide a significant improvement of theaccuracy with which the lithographic apparatus may project a patternonto a substrate.

It is not necessary for the radiation to travel through the absorbermaterial 2, and for some absorber materials 2 substantially allradiation may be absorbed in the absorber material 2. Without being heldto any particular theory, it is believed that the thickness of theabsorbing material 2 generates some kind of waveguides in the gaps 3.The dimensions of these waveguides seem to determine whether there arefocus differences or image shift differences (explained further below)related to the pitches of a pattern on a mask. As such it seems of minorimportance whether or not the absorber material 2 transmits anyimpinging radiation.

It has been the general belief in the lithographic industry thatunwanted phase offsets caused by mask topography, and focus differencecaused by those phase offsets, would reduce in size as the thickness ofthe absorbing material on the mask was reduced. This arose from theunderstanding that the size of unwanted phase offsets increased as thesize of the mask topography variations increased, and that reduced masktopography variation was best achieved through using a thinner layer ofabsorbing material. However, surprisingly, this is not the case.Instead, the phase offsets caused by mask topography increase as thethickness of the absorbing material is reduced (if the absorbingmaterial is thinner than the wavelength of the radiation beam, as is thecase in conventional masks). Furthermore, surprisingly, unwanted phaseoffsets tend towards a minimum (which may be zero) as the thickness ofthe absorbing material tends towards the wavelength of the radiationbeam in the absorbing material. Correspondingly, the focus differencealso tends towards a minimum (which may be zero) as the thickness of theabsorbing material tends towards the wavelength of the radiation beam inthe absorbing material. Hence, in an embodiment of the invention, a maskis provided which has absorbing material 2 having a thickness whichcorresponds with the wavelength of the radiation beam PB in theabsorbing material.

FIG. 3 is a graph which shows the results of a simulation which measureshow the best focus of a lithographic apparatus changes as a function ofthe thickness of the absorbing material of a mask. The term ‘best focus’may be interpreted as meaning the plane in which an aerial image withthe best contrast is obtained. The simulation modelled the projection ofstructures by a lithographic apparatus, and was performed usingHyperlith software, which is available from Panoramic Technology, Inc(www.panoramictech.com). The projection of structures onto a range ofdifferent planes by the lithographic apparatus was simulated. Thevariation of the critical dimension of those projected structures wasthen determined as a function of the plane position in order to generateBossung curves. For a given structure, a maximum or minimum of theBossung curve was taken as indicating the best focus of that structure.The simulation was repeated for different thicknesses of mask absorbingmaterial.

The simulation used a radiation wavelength of 193 nanometers and anabsorbing material refractive index of 1.4. Both axes of the graphindicate units measured in nanometers. Due to the nature of thesimulation, the zero point on the best focus axis is at an arbitrarylocation. The simulation was performed for patterns having fourdifferent pitches, namely 270 nanometers, 135 nanometers, 112.5nanometers and 90 nanometers. The pitch dimensions are the pitches atthe substrate side of the projection system PS (see FIG. 1) of thelithographic apparatus as is conventional. In contrast to this, theabsorbing material thickness indicated on the horizontal axis of thegraph is measured on the mask side of the projection system PS.

As may be seen from FIG. 3, the best focus for patterns having pitchesof 270 nanometers, 135 nanometers and 112.5 nanometers all intersectwith each other when the absorbing material has a particular thickness(the intersection is marked by a dotted vertical line). Thisintersection indicates that the focus difference for those three pitchesis zero when the absorbing material has that thickness. The absorbingmaterial thickness at which the intersection occurs is approximately 130nanometers. The wavelength of radiation passing through the absorbingmaterial is 138 nanometers (=193/1.4). The thickness of the absorbingmaterial which provides a minimized focus difference is thus well within10% of the radiation wavelength, and may be considered to besubstantially equal to the radiation wavelength. At least some of thedifference may arise due to limitations of the simulation. These mayinclude uncertainty when extracting the best focus from a set ofmeasurements. Using curve fitting to fit curves to sets of results mayimprove the extent to which the results of the simulation correspondwith the thickness as calculated using the radiation wavelength and therefractive index of the absorbing material.

The 90 nanometer pitch pattern behaves slightly differently to the otherpitches. As the thickness of the absorbing material increases, the bestfocus for the 90 nanometer pitch draws closer to the best focus forother pitches. However, the best focus does not intersect with the otherbest focuses, thus indicating that a focus difference remains. Thereason why the 90 nanometer pitch line does not intersect with otherpitch lines is not understood, but it may arise from limitations of thesimulation used to generate the graph.

FIG. 4 is a graph which shows the results of a simulation whichcorresponds with the simulation used to generate FIG. 3, except that theabsorbing material had a refractive index of 1.9 rather than 1.4. As maybe seen from FIG. 4, the focus difference behaves in the same manner asa function of absorbing material thickness. That is, the focusdifference reduces as the thickness of the absorbing material increases,and the focus difference passes through a minimum where the lines forpitches of 270 nanometers, 135 nanometers and 112.5 nanometers intersectwith each other (the intersection is marked by a dotted vertical line).This occurs when the absorbing material has a thickness of approximately100 nanometers. The wavelength of radiation passing through theabsorbing material is 102 nanometers (=193/1.9). The thickness of theabsorbing material which provides a minimized focus difference is thuswell within 10% of the radiation wavelength, and may be considered to besubstantially equal to the radiation wavelength. Again, using curvefitting to fit curves to sets of results may improve the extent to whichthe results of the simulation correspond with the thickness ascalculated using the radiation wavelength and the refractive index ofthe absorbing material.

The 90 nanometer pitch pattern again behaves slightly differently to theother pitches. The focus difference reduces as the thickness of theabsorbing material increases, but does not intersect with the focusdifferences measured for other pitches. The reason for this is notunderstood, but it may be due to limitations of the simulation used togenerate the graph.

FIG. 5 shows the results of another simulation which corresponds withthe previous simulations except that the refractive index of theabsorbing material was 2.3. Similar results are seen, with the focusdifference reducing as the thickness of the absorbing material isincreased. In this simulation, an intersection occurs for all pitches(i.e. including 90 nanometers). The focus difference is minimized whenthe absorbing material has a thickness of approximately 75 nanometers(the intersection is marked by a dotted vertical line). The wavelengthof radiation passing through the absorbing material is 84 nanometers(=193/2.3). The thickness of the absorbing material which provides aminimized focus difference is within approximately 10% of the radiationwavelength, and may be considered to be substantially equal to theradiation wavelength. At least some of the difference may arise due tolimitations of the simulation. Furthermore, using curve fitting to fitcurves to sets of results may improve the extent to which the results ofthe simulation correspond with the thickness as calculated using theradiation wavelength and the refractive index of the absorbing material.

The mask absorbing material thicknesses which provide reduced focusdifference (e.g. minimized focus difference, e.g. zero focus difference)are markedly different from conventionally used mask absorbing materialthicknesses. For example, an absorbing material with a refractive indexof 1.4 would conventionally be provided with a thickness of around 43nanometers on a mask. For example, an absorbing material with arefractive index of 1.9 would conventionally be provided with athickness of around 50 nanometers on a mask. However, both of thesethicknesses may give rise to considerable topography induced focusdifference, as has been demonstrated by the simulation results shown inFIGS. 3 and 4. The topography induced focus difference may be reduced byproviding the absorbing material with a significantly increasedthickness compared with a conventionally provided thickness.

In the above described embodiments, the topography induced focusdifferences are reduced by increasing the thickness of the absorbingmaterial beyond the conventional thickness. Additionally oralternatively, dopant may be added to the absorbing material whichincreases its refractive index. Similarly, the refractive index of theabsorbing material may be modified by changing the relative proportionsof molybdenum and silicide in the absorbing material. Any combination ofthese approaches may be used.

In the above described embodiments, the absorbing material is a singlematerial. However, the absorbing material may be more than one material.The materials may, for example, be provided as layers, and may, forexample, be provided as a stack of alternating layers. Simulation hasshown that if the absorbing material is more than one material, then thethickness of the material which will give the best focus difference(e.g. a zero or minimum focus difference) may be determined using theaverage refractive index of the absorbing material. The average of therefractive index takes into account the proportions of the differentmaterials through which the radiation beam passes. For example, if halfof the thickness of the absorbing material is material having arefractive index of 1.4 and half of the thickness of the absorbingmaterial is material having a refractive index of 2.4, then therefractive index of the absorbing material may be treated as being(1.4+2.4)/2=1.9. The refractive index 1.9 may be used to determine thethickness of absorbing material that should be provided in this case.For example, if two thirds of the thickness of the absorbing material ismaterial having a refractive index of 1.4 and one third of the thicknessof the absorbing material is material having a refractive index of 2.4,then the refractive index of the absorbing material may be treated asbeing [(1.4×2)+2.4)]/3=1.7.

FIG. 6 shows the results of a simulation which corresponds withpreviously described simulations, except that half of the thickness ofthe absorbing material has a refractive index of 1.4 and half of thethickness of the absorbing material has a refractive index of 2.4. Asmay be seen, the focus difference reduces as the thickness of theabsorbing material increases, and the focus difference passes through aminimum where the lines for pitches of 112.5 nanometers and 90nanometers intersect with each other (the intersection is marked by adotted vertical line). This occurs when the absorbing material has athickness of approximately 95 nanometers. The wavelength of radiationpassing through the absorbing material is 102 nanometers (=193/1.9). Thethickness of the absorbing material which provides a minimized focusdifference is thus well within 10% of the radiation wavelength, and maybe considered to be substantially equal to the radiation wavelength.

The simulation of FIG. 6 used a single layer of material with arefractive index of 1.4 and a single layer of material with a refractiveindex of 2.4. However, the average refractive index of the absorbingmaterial may be used to determine the thickness of absorbing materialthat should be provided, irrespective of the number of layers ofmaterial which are used to form the absorbing material. The averagerefractive index may, for example, be used even if a stack ofalternating layers of two materials is used to form the absorbingmaterial (or a stack comprising more than two materials).

The above described embodiments of the invention have been directedtowards reducing or eliminating focus difference. Focus differencearises when a radiation beam of a lithographic apparatus suffers evenorder aberrations (e.g. caused by the topography of the mask). When aradiation beam of a lithographic apparatus suffers odd orderaberrations, a pattern image may move in a direction transverse to anoptical axis of the lithographic apparatus. This may be referred to asimage shift. Image shift for an image of a first type of feature (e.g.dense lines) may be different to image shift for an image of a secondtype of feature (e.g. isolated lines) lines. Embodiments of theinvention may reduce the difference between image shifts for differenttypes of features (or features having different pitches) in the same waythat they reduce focus difference. That is, image shift may be reducedby providing the absorbing material with a thickness which reduces oreliminates unwanted phase offset. Image shift may be reduced byproviding the absorbing material with a thickness which is substantiallyequal to the wavelength of the radiation beam in the absorbing material.Image shift may be reduced by providing the absorbing material with athickness which is within 10% of the wavelength of the radiation beam inthe absorbing material. Image shift may be reduced by providing theabsorbing material with a thickness which is within 20% of thewavelength of the radiation beam in the absorbing material.

In an embodiment, the mask may be created by etching a pattern into amask blank. When etching the pattern into the mask blank, regions of theradiation absorbing material which are not etched through to thesubstrate may nevertheless be made thinner by the etching. This thinningmay be taken into account when determining what thickness of radiationabsorbing material to provide on the mask blank. The mask blank may beprovided with a layer of radiation absorbing material which has athickness that is such that, after etching, the thickness of theradiation absorbing material is substantially equal to the certainwavelength divided by a refractive index of the radiation absorbingmaterial. Calculation of the thickness of radiation absorbing materialto provide on the mask blank may take properties of the etching intoaccount (e.g. the duration of the etching).

In an embodiment, a mask may be provided with a functional pattern (i.e.a pattern which will form part of an operational device) and may inaddition be provided with a measurement pattern which does not form partof the functional pattern. The measurement pattern may be, for example,located to one side of the functional pattern. The measurement patternmay be used, for example, to measure alignment of the mask relative tothe substrate table WT (see FIG. 1) of the lithographic apparatus, ormay be used to measure some other parameter. The absorbing materialwhich is used to form the measurement pattern may be different from theabsorbing material which is used to form the functional pattern. Forexample, the absorbing material of the measurement pattern may amaterial which provides substantially complete absorption of theradiation beam. The absorbing material which is used to form themeasurement pattern may be provided with a different thickness than theabsorbing material used to form the functional pattern. The thickness ofthe absorbing material which is used to form the measurement pattern maybe determined using an embodiment of the invention.

The extent to which the radiation beam PB is absorbed by the absorbingmaterial may be different for different masks. For example, theradiation beam PB may be partially absorbed as it travels through theabsorbing material. Alternatively, the radiation beam PB may besubstantially fully absorbed as it passes through the absorbingmaterial, i.e. the absorbing material blocks the radiation beam. A maskwhich has absorbing material that blocks the radiation beam may bereferred to as a binary mask.

In embodiments in which the radiation beam is partially absorbed by theabsorbing material of a mask, the phase of the radiation beam as itexits the absorbing material may affect the contrast of an aerial imageformed using the mask. The contrast may, for example, be at a maximum ifthe phase of radiation which has passed through the absorbing materialis 90° different from the phase of radiation which has not passedthrough the absorbing material. Since the phase of the radiation dependsupon the thickness of the absorbing material, selecting an absorbingmaterial thickness using the approach described above may reduce thecontrast of the aerial image formed using the mask. In some applicationareas this may not be a significant concern. For example, if thelithographic apparatus is being used to image patterns which will formlogic circuits then contrast may be considered to be less important thanfocus difference. The benefit provided by an improvement of focusdifference (e.g. better critical density uniformity) may be consideredto outweigh the reduced contrast.

In an embodiment, the phase shift provided by the mask, and the contrastthat this provides, may be taken into account as well as the masktopography induced focus difference when selecting an absorbing materialthickness. A compromise may be found which provides a necessary degreeof contrast while providing a reduced mask topography induced focusdifference and/or image shift (e.g. compared with a conventionalthickness of absorbing material).

In an embodiment, an antireflection layer may be provided on top of theabsorbing material. The antireflection layer may, for example, have athickness of around 2 nanometers. The antireflection layer is notconsidered to form part of the absorbing material, and has therefore notbeen included when values of the thickness of the absorbing materialhave been stated. In general, a material which is significantlyabsorbing of the radiation beam may be taken into account whendetermining the thickness of the absorbing material.

In an embodiment, the thickness of the absorbing material which providesa minimized focus difference may be within 20% of the radiationwavelength. This may provide a significant reduction of focus differenceand/or image shift compared with providing the absorbing material with aconventional thickness.

Although embodiments of the invention have been described in relation toa transmissive mask (i.e. a mask which transmits radiation), anembodiment of the invention may be applied to a reflective mask (i.e. amask which reflects radiation); In an embodiment in which the mask is areflective mask, the mask may be arranged such that the radiation beamis incident upon absorbing material and gaps, and then passes throughthese to be incident upon a reflector located behind the absorbingmaterial and gaps. In an embodiment in which the mask is a reflectivemask, the absorbing material has a thickness which is substantiallyequal to or a multiple of the certain wavelength divided by twice arefractive index of the absorbing material, after taking an offset intoaccount.

FIG. 7 is a graph which shows the results of a simulation which measureshow the best focus of a lithographic apparatus changes as a function ofthe thickness of the absorbing material of a reflective mask. Thesimulation was performed using Hyperlith software in the mannerdescribed further above. The simulation used a radiation wavelength of13.5 nanometers. The refractive index of the absorbing material wasapproximately 9.5. The vertical axis of the graph shows the best focusand indicates units measured in microns. Due to the nature of thesimulation, the zero point on the best focus axis is at an arbitrarylocation. The horizontal axis of the graph shows the thickness(expressed as height) of the absorbing material, and indicates unitsmeasured in nanometers. The simulation was performed for patterns havinga range of different pitches which extended from a minimum of 36nanometers to a maximum of 120 nanometers. The pitch dimensions are thepitches at the substrate side of the projection system of thelithographic apparatus as is conventional. The absorbing materialthickness is measured on the mask side of the projection system PS.

As may be seen from FIG. 7, the difference between the best focuses fordifferent pitches (the best focus range) increases and decreases in aperiodic manner. In addition, there is a general trend towards a reducedbest focus difference as the thickness of the absorbing material isincreased. FIG. 8 is a graph which shows how the best focus range variesas a function of absorbing material thickness. FIG. 8 confirms that thebest focus range increases and decreases in a periodic manner and trendstowards a reduced best focus difference as the thickness of theabsorbing material increases.

The period of the best focus range variation is approximately 7nanometers. Thus, the focus range passes through a minimum forapproximately every 7 nanometers of thickness of the absorbing material.However, the results of the simulation also include an offset, which mayneed to be taken into account in order to determine a thickness ofabsorbing material that provides a focus range minimum. In this examplethe offset is estimated at around 1-2 nanometers, but may have someother value. The offset may arise in part because the radiation beam isnot perpendicularly incident upon the mask but instead has an angle ofincidence of, for example, 6°. The offset may change as the angle ofincidence of the radiation beam on the mask is changed. The offset mayarise in part because the radiation beam is not reflected from a singlereflective surface of the mask, but instead is reflected from amultilayer structure and penetrates into the multilayer structure.

FIG. 9 shows the data of FIG. 8 divided by the wavelength of theradiation and multiplied by the refractive index of the absorbingmaterial. As can be seen from FIG. 9, the period of the best focus rangeis 0.5, and there is an offset of around 0.1. The period of 0.5 confirmsthat there are two absorbing material thicknesses for each wavelength ofthe radiation in the absorbing material which will provide a best focusminimum.

Although the above relates to minimising the best focus range viaselection of the absorbing material thickness, a corresponding approachmay be used to minimise image shift difference via selection of theabsorbing material thickness.

It may be advantageous to use embodiments of the invention when theradiation beam PB is polarized. If the radiation beam is not polarizedthen the different polarizations which make up the radiation beam maycancel out the mask topography induced focus difference such thatsignificant mask topography induced focus difference is not seen. If theradiation beam is polarized then this cancelling out will not occur, andan embodiment of the invention may be used to reduce mask topographyinduced focus difference. Polarized radiation is conventionally used inimmersion lithography, and embodiments of the invention may therefore beadvantageously used for immersion lithography. The radiation beam of anEUV lithographic apparatus may have a main angle, for example, of around6°, and as a result different polarisation states provide differentcontributions to the radiation beam. Consequently, the reflected beam isdifferent for the two polarization directions and as such can beconsidered to be polarized (at least to some extent). Embodiments of theinvention may therefore be advantageously used for EUV lithography.

Although embodiments of the invention have been described in relation toradiation of 193 nanometers, an embodiment of the invention may be usedin connection with other wavelengths. These may include for exampleother ultraviolet (UV) radiation (e.g. having a wavelength of 365, 248,157 or 126 nm) and/or extreme ultra-violet (EUV) radiation (e.g. havinga wavelength in the range of 5-20 nm). Similarly, although embodimentsof the invention have been described in connection with immersionlithography, an embodiment of the invention may be used in connectionwith any form of projection lithography (e.g. non-immersionlithography).

The mask shown in FIGS. 1 and 2 may be referred to as a lithographicmask. The term ‘lithographic mask’ may be interpreted as meaning a maskwhich is suitable for use in a lithographic apparatus.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention. For example, the embodiments of the invention may take theform of a computer program containing one or more sequences ofmachine-readable instructions describing a method as disclosed above, ora data storage medium (e.g. semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein. Further, themachine-readable instruction may be embodied in two or more computerprograms. The two or more computer programs may be stored on one or moredifferent memories and/or data storage media.

The invention may further be described using the following clauses:

1. A lithographic mask comprising a substrate which is substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the absorbing material has a thicknesswhich is substantially equal to the certain wavelength divided by arefractive index of the absorbing material.2. A lithographic mask comprising a substrate which is substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the absorbing material has a thicknessequal to or within a first range of 10% of the radiation wavelengthdivided by a refractive index of the absorbing material.3. A lithographic mask comprising a substrate which is substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the absorbing material has a thicknessequal to or within a second range of 20% of the radiation wavelengthdivided by a refractive index of the absorbing material.4. The lithographic mask according to clause 2 or clause 3, wherein thethickness of the absorbing material within the first range or the secondrange is further chosen depending on a phase shift characteristic andresulting image contrast provided by the absorbing material to radiationof the certain wavelength.5. The lithographic mask according to clause 4, wherein the thickness ofthe absorbing material is chosen to optimize the phase shiftcharacteristic and resulting image contrast provided by the absorbingmaterial to radiation of the certain wavelength.6. The lithographic mask according to any of the previous clauses,wherein the absorbing material comprises a plurality of material layershaving different refractive indices, and wherein the refractive index ofthe absorbing material is taken as being the average of the refractiveindices of the material layers, the average taking into account theproportions of the different materials through which the radiation beampasses.7. The lithographic mask according to any of the previous clauses,wherein the lithographic mask comprises a first arrangement having afirst radiation absorbing material having a first thickness, the firstthickness being determined according to any of the previous clauses andthe lithographic mask further comprises a second arrangement having asecond radiation absorbing material having a second thickness, the firstthickness and the second thickness being different.8. The lithographic mask according to clause 7, wherein the firstarrangement is a functional pattern and the second arrangement is ameasurement pattern.9. The lithographic mask of clause 6 or clause 7, wherein the firstradiation absorbing material is the same material as the secondradiation absorbing material.10. The lithographic mask of any preceding clause, wherein the certainwavelength is one of 193 nm, 365 nm, 248 nm, 157 nm or 126 nm.11. A lithographic mask comprising a reflective substrate having aradiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofa certain wavelength, wherein the absorbing material has a thicknesswhich is substantially equal to or a multiple of the certain wavelengthdivided by twice a refractive index of the absorbing material, aftertaking an offset into account.12. A method comprising:

providing a beam of radiation with a certain wavelength using anillumination system;

using a mask to impart the radiation beam with a pattern in itscross-section; and

projecting the patterned radiation beam onto a target portion of asubstrate,

wherein the mask comprises a substrate transmissive to the radiationbeam and the substrate has a radiation absorbing material provided in anarrangement, and wherein the thickness of the absorbing material issubstantially equal to the radiation wavelength divided by a refractiveindex of the absorbing material.

13. A lithographic mask comprising a substrate substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the radiation absorbing materialarrangement comprises a plurality of structures having a first pitch anda plurality of structures having a second pitch different compared tothe first pitch, and wherein the absorbing material has a thicknesswhich is such that the structures having the first pitch and thestructures having the second pitch will have substantially equal bestfocus planes when projected using a projection system of a lithographicapparatus.14. A lithographic mask comprising a substrate substantiallytransmissive for radiation of a certain wavelength, the substrate havinga radiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the radiation absorbing materialarrangement comprises a plurality of structures having a first pitch anda plurality of structures having a second pitch different compared tothe first pitch, and wherein the absorbing material has a thicknesswhich is such that the structures having the first pitch and thestructures having the second pitch will experience substantially equalimage shifts when projected using a projection system of a lithographicapparatus.15. A lithographic mask comprising a reflective substrate having aradiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beamhaving a certain wavelength, wherein the radiation absorbing materialarrangement comprises a plurality of structures having a first pitch anda plurality of structures having a second pitch different compared tothe first pitch, and wherein the absorbing material has a thicknesswhich is such that a focus difference between the best focus plane forthe structures having the first pitch and a best focus plane for thestructures having the second pitch substantially corresponds to aminimum of a variation of the focus difference versus absorbing materialthickness when projected using a projection system of a lithographicapparatus.16. A lithographic mask comprising a reflective substrate having aradiation absorbing material in an arrangement, the arrangementconfigured to apply a pattern to a cross-section of a radiation beam ofthe certain wavelength, wherein the radiation absorbing materialarrangement comprises a plurality of structures having a first pitch anda plurality of structures having a second pitch different compared tothe first pitch, and wherein the absorbing material has a thicknesswhich is such that an image shift difference between the image shiftexperienced by the structures having the first pitch and the image shiftexperience by the structures having the second pitch substantiallycorresponds to a minimum of a variation of the image shift differenceversus absorbing material thickness when projected using a projectionsystem of a lithographic apparatus.17. The lithographic mask according to any of clauses 13 to 16, whereinthe first pitch, as measured after projection by the projection systemof the lithographic apparatus, is substantially half of the wavelengthof the certain wavelength.18. The lithographic mask according to any of clauses 13 to 16, whereinthe first pitch, as measured after projection by the projection systemof the lithographic apparatus, is smaller than the certain wavelengthand the second pitch, as measured after projection by the projectionsystem of the lithographic apparatus, is larger than the certainwavelength.19. A blank mask comprising a substrate substantially transmissive forradiation of a certain wavelength, the substrate having a radiationabsorbing material, the radiation absorbing material being etchable togenerate a pattern in the radiation absorbing material, wherein athickness of the radiation absorbing material is such that followingetching the radiation absorbing material has a thickness which issubstantially equal to the certain wavelength divided by a refractiveindex of the radiation absorbing material.20. A blank mask comprising a reflective substrate having a radiationabsorbing material, the radiation absorbing material being etchable togenerate a pattern in the radiation absorbing material, wherein athickness of the radiation absorbing material is such that followingetching the radiation absorbing material has a thickness whichsubstantially corresponds to a minimum of a variation of the focusdifference versus absorbing material thickness when projected at acertain wavelength using a projection system of a lithographicapparatus.21. The blank mask according to clause 19 or clause 20, wherein a layerof a radiation sensitive resist is provided over the radiation absorbingmaterial.22. A method of determining a thickness of a radiation absorbingmaterial to be provided to a lithographic mask, the lithographic maskcomprising a substrate substantially transmissive for radiation of acertain wavelength and the substrate comprising the radiation absorbingmaterial arranged to form a plurality of structures for imaging via aprojection system of a lithographic apparatus, the method comprising:

-   -   selecting a plurality of structures having different pitches,    -   determining best focus planes of the selected structures when        imaged via the projection system, the best focus planes being        determined for different thicknesses of the radiation absorbing        material, and    -   selecting the thickness of the radiation absorbing material for        which the plurality of structures have substantially equal best        focus planes when imaged using the projection system.        23. A method of determining a thickness of a radiation absorbing        material to be provided to a lithographic mask, the lithographic        mask comprising a substrate substantially transmissive for        radiation of a certain wavelength and the substrate comprising        the radiation absorbing material arranged to form a plurality of        structures for imaging via a projection system of a lithographic        apparatus, the method comprising:    -   selecting a plurality of structures having different pitches,    -   determining image shifts of the selected structures when imaged        via the projection system, the image shifts being determined for        different thicknesses of the radiation absorbing material, and    -   selecting the thickness of the radiation absorbing material for        which the plurality of structures have substantially equal image        shifts when imaged using the projection system.        24. A method of determining a thickness of a radiation absorbing        material to be provided to a lithographic mask, the lithographic        mask comprising a reflective substrate and the substrate        comprising the radiation absorbing material arranged to form a        plurality of structures for imaging using a projection system of        a lithographic apparatus, the method comprising:    -   selecting a plurality of structures having different pitches,    -   determining best focus planes of the selected structures when        imaged via the projection system using a certain wavelength, the        best focus planes being determined for different thicknesses of        the radiation absorbing material, and    -   selecting the thickness of the radiation absorbing material to        be a thickness at which a variation, as a function of absorbing        material thickness, of a difference between best focus planes of        the selected structures substantially corresponds to a minimum        when the structures are imaged using the projection system.        25. A method of determining a thickness of a radiation absorbing        material to be provided to a lithographic mask, the lithographic        mask comprising a reflective substrate and the substrate        comprising the radiation absorbing material arranged to form a        plurality of structures for imaging using a projection system of        a lithographic apparatus, the method comprising:    -   selecting a plurality of structures having different pitches,    -   determining image shifts of the selected structures when imaged        via the projection system using a certain wavelength, the image        shifts being determined for different thicknesses of the        radiation absorbing material, and    -   selecting the thickness of the radiation absorbing material to        be a thickness at which a variation, as a function of absorbing        material thickness, of a difference between image shifts of the        selected structures substantially corresponds to a minimum when        the structures are imaged using the projection system.        26. The method according to any of clauses 22 to 25, wherein the        determining the best focus plane or the determining the image        shift is performed by simulating the projection of the        structures using the projection system.        27. The method according to any of clauses 22 to 26, wherein a        first structure in the plurality of structures comprises a first        pitch, as measured after projection by the projection system,        having a dimension of substantially half of the wavelength of        the certain wavelength.        28. The method according to any of clauses 22 to 26, wherein a        first structure in the plurality of structures comprises a pitch        being smaller than the certain wavelength, and wherein a second        structure in the plurality of structures comprises a pitch being        larger than the certain wavelength, the pitches being as        measured after projection by the projection system.        29. The method according to any of clauses 22 to 28, wherein the        selecting the thickness comprises defining a range within which        the thickness is chosen.        30. The method according to clause 29, wherein the selecting the        thickness further comprises selecting the thickness of the        absorbing material depending on a phase-shifting characteristic        of the absorbing material to the radiation of the certain        wavelength.        31. The method according to clause 30, wherein the thickness of        the absorbing material is further selected to optimize the        phase-shifting characteristic of the absorbing material.        32. A computer program product configured to perform the method        according to any of clauses 22 to 31.        33. A lithographic apparatus comprising:

a mask according to any preceding clause

a support structure to support the mask, the mask serving to impart aradiation beam with a pattern in its cross-section;

a substrate table to hold a substrate; and

a projection system to project the patterned radiation beam onto atarget portion of the substrate.

34. The lithographic apparatus according to clause 33, wherein theradiation beam is a polarized radiation beam.35. The lithographic apparatus according to clause 34, wherein thepolarized radiation beam has at least two polarization states, anintensity in a first polarization state being different from anintensity in a second polarization state.36. A method comprising:

providing a beam of radiation with a certain wavelength using anillumination system;

using a mask according to any preceding clause to impart the radiationbeam with a pattern in its cross-section; and

projecting the patterned radiation beam onto a target portion of asubstrate.

It will be appreciated that aspects of the present invention can beimplemented in any convenient way including by way of suitable hardwareand/or software. Alternatively, a programmable device may be programmedto implement embodiments of the invention. The invention therefore alsoprovides suitable computer programs for implementing aspects of theinvention. Such computer programs can be carried on suitable carriermedia including tangible carrier media (e.g. hard disks, CD ROMs and soon) and intangible carrier media such as communications signals.

1.-9. (canceled)
 10. A lithographic mask comprising a substratesubstantially transmissive for radiation of a certain wavelength, thesubstrate having a radiation absorbing material in an arrangement, thearrangement configured to apply a pattern to a cross-section of aradiation beam of the certain wavelength, wherein the radiationabsorbing material arrangement comprises a plurality of structureshaving a first pitch and a plurality of structures having a second pitchdifferent compared to the first pitch, and wherein the absorbingmaterial has a thickness which is such that the structures having thefirst pitch and the structures having the second pitch will havesubstantially equal best focus planes when projected using a projectionsystem of a lithographic apparatus.
 11. The lithographic mask accordingto claim 10, wherein the first pitch, as measured after projection bythe projection system of the lithographic apparatus, is smaller than thecertain wavelength and the second pitch, as measured after projection bythe projection system of the lithographic apparatus, is larger than thecertain wavelength.
 12. A lithographic mask comprising a substratesubstantially transmissive for radiation of a certain wavelength, thesubstrate having a radiation absorbing material in an arrangement, thearrangement configured to apply a pattern to a cross-section of aradiation beam of the certain wavelength, wherein the radiationabsorbing material arrangement comprises a plurality of structureshaving a first pitch and a plurality of structures having a second pitchdifferent compared to the first pitch, and wherein the absorbingmaterial has a thickness which is such that the structures having thefirst pitch and the structures having the second pitch will experiencesubstantially equal image shifts when projected using a projectionsystem of a lithographic apparatus.
 13. The lithographic mask accordingto claim 12, wherein the first pitch, as measured after projection bythe projection system of the lithographic apparatus, is smaller than thecertain wavelength and the second pitch, as measured after projection bythe projection system of the lithographic apparatus; is larger than thecertain wavelength.
 14. A lithographic mask comprising a reflectivesubstrate having a radiation absorbing material in an arrangement, thearrangement configured to apply a pattern to a cross-section of aradiation beam having a certain wavelength, wherein the radiationabsorbing material arrangement comprises a plurality of structureshaving a first pitch and a plurality of structures having a second pitchdifferent compared to the first pitch, and wherein the absorbingmaterial has a thickness which is such that a focus difference betweenthe best focus plane for the structures having the first pitch and abest focus plane for the structures having the second pitchsubstantially corresponds to a minimum of a variation of the focusdifference versus absorbing material thickness when projected using aprojection system of a lithographic apparatus.
 15. The lithographic maskaccording to claim 14, wherein the first pitch, as measured afterprojection by the projection system of the lithographic apparatus, issmaller than the certain wavelength and the second pitch, as measuredafter projection by the projection system of the lithographic apparatus,is larger than the certain wavelength.
 16. A lithographic maskcomprising a reflective substrate having a radiation absorbing materialin an arrangement, the arrangement configured to apply a pattern to across-section of a radiation beam of the certain wavelength, wherein theradiation absorbing material arrangement comprises a plurality ofstructures having a first pitch and a plurality of structures having asecond pitch different compared to the first pitch, and wherein theabsorbing material has a thickness which is such that an image shiftdifference between the image shift experienced by the structures havingthe first pitch and the image shift experience by the structures havingthe second pitch substantially corresponds to a minimum of a variationof the image shift difference versus absorbing material thickness whenprojected using a projection system of a lithographic apparatus.
 17. Thelithographic mask according to claim 16, wherein the first pitch, asmeasured after projection by the projection system of the lithographicapparatus, is substantially half of the wavelength of the certainwavelength.
 18. The lithographic mask according to claim 16, wherein thefirst pitch, as measured after projection by the projection system ofthe lithographic apparatus, is smaller than the certain wavelength andthe second pitch, as measured after projection by the projection systemof the lithographic apparatus, is larger than the certain wavelength.19.-21. (canceled)
 22. A method of determining a thickness of aradiation absorbing material to be provided to a lithographic mask, thelithographic mask comprising a substrate substantially transmissive forradiation of a certain wavelength and the substrate comprising theradiation absorbing material arranged to form a plurality of structuresfor imaging via a projection system of a lithographic apparatus, themethod comprising: selecting a plurality of structures having differentpitches, determining best focus planes of the selected structures whenimaged via the projection system, the best focus planes being determinedfor different thicknesses of the radiation absorbing material, andselecting the thickness of the radiation absorbing material for whichthe plurality of structures have substantially equal best focus planeswhen imaged using the projection system.
 23. The method according toclaim 22, wherein the first pitch, as measured after projection by theprojection system of the lithographic apparatus, is smaller than thecertain wavelength and the second pitch, as measured after projection bythe projection system of the lithographic apparatus, is larger than thecertain wavelength.
 24. A method of determining a thickness of aradiation absorbing material to be provided to a lithographic mask, thelithographic mask comprising a substrate substantially transmissive forradiation of a certain wavelength and the substrate comprising theradiation absorbing material arranged to form a plurality of structuresfor imaging via a projection system of a lithographic apparatus, themethod comprising: selecting a plurality of structures having differentpitches, determining image shifts of the selected structures when imagedvia the projection system, the image shifts being determined fordifferent thicknesses of the radiation absorbing material, and selectingthe thickness of the radiation absorbing material for which theplurality of structures have substantially equal image shifts whenimaged using the projection system.
 25. The method according to claim24, wherein the first pitch, as measured after projection by theprojection system of the lithographic apparatus, is smaller than thecertain wavelength and the second pitch, as measured after projection bythe projection system of the lithographic apparatus, is larger than thecertain wavelength.
 26. A method of determining a thickness of aradiation absorbing material to be provided to a lithographic mask, thelithographic mask comprising a reflective substrate and the substratecomprising the radiation absorbing material arranged to form a pluralityof structures for imaging using a projection system of a lithographicapparatus, the method comprising: selecting a plurality of structureshaving different pitches, determining best focus planes of the selectedstructures when imaged via the projection system using a certainwavelength, the best focus planes being determined for differentthicknesses of the radiation absorbing material, and selecting thethickness of the radiation absorbing material to be a thickness at whicha variation, as a function of absorbing material thickness, of adifference between best focus planes of the selected structuressubstantially corresponds to a minimum when the structures are imagedusing the projection system.
 27. The method according to claim 26,wherein the first pitch, as measured after projection by the projectionsystem of the lithographic apparatus, is smaller than the certainwavelength and the second pitch, as measured after projection by theprojection system of the lithographic apparatus, is larger than thecertain wavelength.
 28. A method of determining a thickness of aradiation absorbing material to be provided to a lithographic mask, thelithographic mask comprising a reflective substrate and the substratecomprising the radiation absorbing material arranged to form a pluralityof structures for imaging using a projection system of a lithographicapparatus, the method comprising: selecting a plurality of structureshaving different pitches, determining image shifts of the selectedstructures when imaged via the projection system using a certainwavelength, the image shifts being determined for different thicknessesof the radiation absorbing material, and selecting the thickness of theradiation absorbing material to be a thickness at which a variation, asa function of absorbing material thickness, of a difference betweenimage shifts of the selected structures substantially corresponds to aminimum when the structures are imaged using the projection system. 29.The method according to claim 28, wherein the first pitch, as measuredafter projection by the projection system of the lithographic apparatus,is substantially half of the wavelength of the certain wavelength. 30.The method according to claim 28, wherein the first pitch, as measuredafter projection by the projection system of the lithographic apparatus,is smaller than the certain wavelength and the second pitch, as measuredafter projection by the projection system of the lithographic apparatus,is larger than the certain wavelength.